PVC Resin Viscosity (K Value): What It Means and How to Read It

Every PVC resin grade is essentially a statement about molecular weight. The two common ways of expressing it are the viscosity number (in ml/g) and the K value (dimensionless). They describe the same underlying property.

Which one the manufacturer publishes

Sanlian publishes the viscosity number in ml/g for the SG grades. This site quotes the same figures, in the same units, for the same reason: quoting a different measure than the manufacturer publishes creates a translation step where errors creep in.

What viscosity actually controls

Mechanical properties. Higher viscosity means higher molecular weight means higher tensile strength and better impact resistance. This is why soft products that need to survive handling use the higher band.

Processing. Higher viscosity takes longer to fuse. The material spends more time in the melt before it degrades, which narrows the processing window. On a rigid extrusion line running a stable schedule, that narrower window is a real cost.

Fusion and surface quality. Lower viscosity fuses faster and gives a better surface finish on rigid profiles. If your product is sold on appearance, this is often the deciding factor.

Solubility and plasticisation. Higher viscosity needs more plasticiser to reach the same softness, which shows up in your formulation cost and in migration figures downstream.

How viscosity is measured

The viscosity number is determined by measuring the dilute-solution viscosity of the polymer and converting it. The practical consequence for you is that it is a lot-to-lot consistency measure, not a single number per grade. That is why the specification is a band and not a point: resin within a band behaves consistently in your process, resin at the edge of the band may not.

Why specify a grade, not a K value

K values are used widely in the industry and are convenient because they are dimensionless and easy to compare across sources. But specifying "K67" when the manufacturer's catalogue is organised by grade number introduces a conversion step, and conversions are only approximate — a K value maps to a viscosity band, not to a single number.

Specifying the grade and grade level the manufacturer publishes avoids the conversion entirely. If you need a K value on your paperwork, we can state it alongside the grade, but the grade is what we quote and what the certificate of analysis reports.

Practical advice

• If you are switching suppliers, compare the viscosity band first, then the indicators that matter for your product.
• If your product is clear, viscosity is not your deciding variable — the impurity count and the fish eye figure are.
• If a grade "suddenly" stops working in your process, check the certificate of analysis against the previous batch before changing the formulation. A drift in viscosity is much easier to fix than a drift you do not know about.

Send us your current grade and your end product and we can tell you where it sits in the range and what to look at if you are changing supplier.